Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
27
The general trend currently is increasing towards the positive phase.
Flohn (1993a, 1993b) reported, that between the periods of 1971-1991 and 19511971 the mean pressure in winter of the Icelandic low deepened by 3.5 hPa and the
mean pressure of the Azorean high and the pressure over the Mediterranean
increased in the average by 2-2.5 hPa. Thus the pressure gradient between about
30
0
N and Iceland steepened by about 4 hPa. Malberg and B6kens (1993) as well
come to the conclusion that the pressure gradient over the north Atlantic and Europe
increased over the time period from 1960 to 1990. They inter alia investigated the
seasonal changes of the pressure gradient over the traverse from the Azorean high
and the Icelandic low (2810 km) for the sixties, seventies and eighties, which is
covering a full cycle of the periodic variability. During winter the gradient changed
in 20 years from 12.9 hPa to 22.5 hPa, or +9.6 hPa, while during summer the
change was rather in the reverse direction, from 14.9 hPa to 13.9 hPa or -1 hPa 3 •
4.3 Synoptic Scales
The interaction between the general circulation and the semi-permanent North
Atlantic pressure systems determine the path of the disturbances imbedded in the
Westerlies primarily during winter and cause an interannual variability of the
Mediterranean climate, specifically its precipitation regime.
During winter vigorous synoptic scale low pressure systems detaching from the
Rossby waves determine weather and climate. High mountain barriers such as the
Atlas, the Pyrenees and the Alps are necessary to modify or generate these systems
which are developing in the middle troposphere and gain their momentum by
internal energy transfer processes. Well known is the Genova cyclone generated by
the interaction of westerly airstream and the bow ofthe Alps. Behind the mountains
often chinook-like pattern develop. Smaller topographic obstacles are less important
for these processes. The temperature during this season over the whole
Mediterranean area including the north-western African coasts to a large degree is
determined by the sea surface temperature. Topographic temperature features almost
disappear in satellite images with the exception of high mountains and the Sahara
(Bolle 1999).
4.4 Processes and Interactions At Meso- to Local Scales
During summer and early autumn the Mediterranean Basin is more decoupled from
the zonal circulation. The westerlies pass further north and only seldom (in
"blocking" situations) affect the Mediterranean basin. Its atmosphere now develops
3 For the traverse Malta to north of Norway (3350 km) the respective numbers are: winter change
from 9.4 hPa to 16.2 hPa, or +6.8 hPa, during summer from 4.1 hPa to 3.9 hPa with a minimum
during the seventies of 3.6 hPa, or in the average, maybe, -0.2 hPa
27
The general trend currently is increasing towards the positive phase.
Flohn (1993a, 1993b) reported, that between the periods of 1971-1991 and 19511971 the mean pressure in winter of the Icelandic low deepened by 3.5 hPa and the
mean pressure of the Azorean high and the pressure over the Mediterranean
increased in the average by 2-2.5 hPa. Thus the pressure gradient between about
30
0
N and Iceland steepened by about 4 hPa. Malberg and B6kens (1993) as well
come to the conclusion that the pressure gradient over the north Atlantic and Europe
increased over the time period from 1960 to 1990. They inter alia investigated the
seasonal changes of the pressure gradient over the traverse from the Azorean high
and the Icelandic low (2810 km) for the sixties, seventies and eighties, which is
covering a full cycle of the periodic variability. During winter the gradient changed
in 20 years from 12.9 hPa to 22.5 hPa, or +9.6 hPa, while during summer the
change was rather in the reverse direction, from 14.9 hPa to 13.9 hPa or -1 hPa 3 •
4.3 Synoptic Scales
The interaction between the general circulation and the semi-permanent North
Atlantic pressure systems determine the path of the disturbances imbedded in the
Westerlies primarily during winter and cause an interannual variability of the
Mediterranean climate, specifically its precipitation regime.
During winter vigorous synoptic scale low pressure systems detaching from the
Rossby waves determine weather and climate. High mountain barriers such as the
Atlas, the Pyrenees and the Alps are necessary to modify or generate these systems
which are developing in the middle troposphere and gain their momentum by
internal energy transfer processes. Well known is the Genova cyclone generated by
the interaction of westerly airstream and the bow ofthe Alps. Behind the mountains
often chinook-like pattern develop. Smaller topographic obstacles are less important
for these processes. The temperature during this season over the whole
Mediterranean area including the north-western African coasts to a large degree is
determined by the sea surface temperature. Topographic temperature features almost
disappear in satellite images with the exception of high mountains and the Sahara
(Bolle 1999).
4.4 Processes and Interactions At Meso- to Local Scales
During summer and early autumn the Mediterranean Basin is more decoupled from
the zonal circulation. The westerlies pass further north and only seldom (in
"blocking" situations) affect the Mediterranean basin. Its atmosphere now develops
3 For the traverse Malta to north of Norway (3350 km) the respective numbers are: winter change
from 9.4 hPa to 16.2 hPa, or +6.8 hPa, during summer from 4.1 hPa to 3.9 hPa with a minimum
during the seventies of 3.6 hPa, or in the average, maybe, -0.2 hPa